The invention concerns an electric motor with a stator having a bore, in which a rotor is arranged, and with a control arrangement having at least one coil with a core, the coil being loaded by the motor current.
The control enables changes in the supply to the motor of electric current and electric voltage, hereby changing the speed or torque. For this purpose, the control arrangement can be a controlled rectifier, a controlled inverter, a frequency converter etc. Usually, the control arrangement is made as power electronics.
Independently of the exact embodiment of the control arrangement, inductivities are often required, which are realised by way of coils. An example is an intermediary circuit coil, which is loaded by the intermediary circuit current, i.e. the current between the rectifier and the inverter, in a frequency converter. This means that the total current flowing through the motor also flows through the intermediary circuit coil. Another example of a coil loaded by the motor current is a filter coil, which is arranged in the input area of the control arrangement with the purpose of reducing the emission of electrical noise signals. Also output and motor coils could be imagined, which are arranged in the output area of the control arrangement to protect the motor against voltage transients, which could damage the isolation of the motor. The motor current also flows through these coils, even when only the current of one phase is concerned.
Common for all these coils is, however, that due to the relatively large current load they emit a substantial amount of heat and are correspondingly large, meaning that they require much room. As in many cases the control arrangement must be integrated in the motor, i.e. form one unit together with the motor, the placing of the coil represents a problem. To solve this problem DE 36 42 724 proposes that the intermediary circuit coil of a frequency converter is placed in the motor housing in a way that it is axially displaced in relation to the stator and the rotor and surrounds an extension of the rotor shaft. The rotor shaft then forms the core of the coil. The intermediary circuit coil is cooled by the air flow from a ventilator, which is also arranged on the rotor shaft.
This solution is room saving, but still the coil takes relatively much room in the construction of this motor. The motor shaft must be extended by at least the coil width, which means an additional increase of the size of the motor. The extension of the rotor shaft may cause stability problems. Additionally, the magnetic flux through the rotor shaft is difficult to control. There is a risk that undesired mutual influences with the magnetic field of the motor may occur.
It is the task of the invention to arrange the coil in a room or space saving way without causing significant interference with the magnetic design of the motor.
In an electric motor as mentioned in the introduction this task is solved in that the core of the coil is arranged on the stator whose iron is used for a magnetic circuit.
The fact that the iron of the stator is used for the magnetic circuit permits realisation of the desired inductivity of the coil, even when only a limited room is available. However, the iron of the stator is available anyway. The coil is arranged on the outside of the stator, i.e. separated from the stator bore. This does not necessarily mean that the core is fitted on the outside of the stator.
It can also be arranged in the stator, if an access from the outside is possible, at least during fitting. As the iron of the stator xe2x80x9cshort-circuitsxe2x80x9d the field lines of both coil and stator winding, a mutual interference does not take place. If a mutual influencing of the fields should occur, it would still be within tolerable frames. As iron is known to be a relatively good heat conductor, the heat produced by the coil can be dissipated relatively fast. It can distribute over the stator, so that the stator surface is available as emitting surface. Also with higher power consumption an overheating is avoided.
In a preferred embodiment the core is part of the stator. Thus the core is made in one piece with the stator, e.g. as a projection of the stator. In this way an air gap is prevented, which would arise by fixing the core on the stator iron, and the magnetic resistance of the magnetic circuit is kept small. The coil can more easily be integrated in the motor.
Preferably, the stator has at least one flux restriction for the separation of the magnetic flux of the coil on one side and of the stator winding on the other side. If the prevention of mutual interference of the fields of coil and stator winding is even more strongly desired, the flux restriction assists in making a measure available, with which this influencing is almost eliminated or at least drastically reduced. The flux restriction increases the magnetic resistance in the surroundings of the coil, and thus screens the field of the coil against the field of the stator winding and vice versa.
In a preferred embodiment the flux restriction is made as a recess in the stator, which is filled with a magnetically non- or poorly conducting material. This recess could e.g. be an air gap. For reasons of stability it could be expedient to arrange several air gaps next to each other. In principle, the remaining ribs in the iron of the stator permit the penetration of the magnetic field. However, these interferences are negligible.
Preferably, a cooling medium can flow through the recess. This could e.g. simply be cooling air. In case that a fluid cooled motor is concerned, this cooling medium could also be oil or another fluid. Thus the recess does not only screen the magnetic field of the coil from that of the stator winding, it also permits the immediate removal of the heat produced in the coil.
Advantageously, the core only extends over part of the axial length of the stator. Thus the coil size can be adapted to the requirements, without having to change the point where the core is arranged. It is thus not necessary to extend the coil over the whole axial length of the core, even though this is of course possible.
Advantageously, in front view the stator has a substantially rectangular cross section and the coil is arranged in the area of a corner of this cross section. To improve assembly or generally the handling of motors, the stator has in many cases a rectangular or even square cross section. It is obvious that in the area of the diagonals, i.e. in the area of the corners of this rectangle or square, the stator has a larger diameter, even though this is often not required for magnetic or electrical purposes. This somehow abundant iron and the room can be used for the coil and its core, respectively, which is very space or room efficient.
Advantageously, the stator and the core are formed with laminated sheet plates. This is especially advantageous with motors operated with AC or three-phase current. In a known way, the formation of eddy-currents is prevented.
Further, it is especially preferred that the core is formed by the same sheet blank tool used for forming the laminated sheets of the stator. It is relatively simple to create the shape or profile of the sheets. Normally, such laminated sheets are punched. Providing the punching tool with a corresponding cutting line will enable a relatively simple creation of the shape of the laminated sheets, so that the core will arise automatically when laminating these sheets to assemble the stator.
In this connection it is advantageous that in the axial direction the stator consists of at least two different types of laminated sheet plates having different shapes, of which one has no core forming area. The laminated sheet plates with core forming area then form the core. However, they only extend over a predetermined axial length of the stator, as not all sheets can be made so. The remaining length is then free of such a core. Here the stator then corresponds to a normal motor.
Advantageously, the coil is fitted on a coil carrier which can be pushed onto the core. This simplifies the production. The coil can be prefitted. E.g. it can be wound on the coil carrier, before the coil carrier is pushed onto the core. The coil carrier can e.g. be made of a synthetic material, so that the magnetic properties are not or not significantly influenced. The hereby occurring distance between coil and core can be kept small enough, when the thickness of the coil carrier is reduced.
In a preferred embodiment it is provided that the core is substantially oriented in the radial direction. This orientation is especially recommended, when the core is arranged in the corner area of the rectangular or square cross section shape. This provides that a relatively large iron area of the stator is available for the magnetic circuit.
It is particularly preferred that the core is arranged in a volume in the stator, which volume is closed by a cover. Thus, the magnetic circuit can be closed by the cover. Any air gaps remaining between cover and stator are so small, that they can be accepted.
In order to additionally reduce or even eliminate such air gaps, the cover can be welded onto the stator. In this case the coil with core is integrated in the stator. This example clearly demonstrates that the stator with coil does not necessarily have larger dimensions than the stator without coil.
In an alternative embodiment the core is substantially parallel to one outside of the stator. This also enables formation of the desired magnetic circuit.
In this connection it is preferred that the core is arranged offset from the outside inwards. Thus sufficient room will be available on the outside for the admission of the coil windings, without having to increase the external dimensions of the stator.
It is also advantageous if the core has the profile of an EI-core. This facilitates the design, as the calculation opportunities of such cores are known.
Preferably, several of the coils each have their own core. These coils can then be distributed around the stator, or, if each coil does not require the total axial length of the stator, they can be arranged in the axial direction. The decision of using one or more coils depends on the design of the machine.
In a particularly preferred embodiment it can be provided that at least two of the coils are part coils of a function coil from the group of intermediary circuit coil, filter coil, noise suppression coil or motor coil, whereby the motor coils are arranged between the motor connections and the control arrangement. In many cases it can be advantageous, for mechanical, magnetic, electrical or thermal reasons, to divide a function coil, which can in an electrical principal circuit diagram be called a concentrated element, into several, physically separated part coils. This is possible by using several coils.
Alternatively, the coils are at least two different function coils from the group of intermediary circuit coil, filter coil, noise suppression coil or motor coil, whereby the motor coils are arranged between the motor connections and the control arrangement. Then, several of the required a * coils can be arranged in the stator, without having to increase the dimensions of the control arrangement and the motor.